An example of a non-destructive method used to inspect defects may include visual inspection, ultrasonic inspection, radiation inspection, magnetic particle inspection, penetrate inspection, eddy current inspection, and the like. The visual inspection and the ultrasonic inspection rely on experience and subjectivity of an examiner, and thus has less reliability and reproducibility and the radiation inspection is difficult to inspect linear defects, such as cracks vertical to a inspection surface, and the like, and the eddy current inspection may inspect defects on a surface of a conductive material or in the vicinity of the surface.
Since a magnetic flux leakage (MFL) method for measuring a magnetic flux leaked due to defects on inner and outer walls of a subject uses a magnetic field, the MFL measuring method may be used only in a structure made of ferromagnetic materials, but may relatively easily perform an inspection, may be less affected by a size, a shape, or the like of an inspection object, and may easily identify a kind of defects.
FIG. 1 illustrates a phenomenon in which a magnetic flux is leaked at a defective site. In FIG. 1, a defect 61 generates a large magnetic resistance when a subject 60 is magnetized. In this case, a bypass magnetic flux is collected around a defect, such that the strength of an effective magnetic field may increase. A large increase in magnetic induction increases the strength of the effective magnetic field. The subject is applied with a tensile stress by an internal stress, such that the defect of the subject may serve as a stress raiser, thereby increasing permeability around a defect.
Therefore, lines of magnetic force flowing in the subject are collected at a lower portion of a defect with increased permeability as illustrated in FIG. 1. In this small region, the lines of magnetic force are behaved like a magnetic line dipole. The dipole has the same direction as the magnetic field disturbed in the subject and generates the magnetic field having an opposite direction to the leakage flux in the defect. Therefore, a defect may be detected by measuring the leakage flux even at a portion at which the defect is present or an opposite portion thereto.
Various types of equipments of detecting the leakage flux as described above have been proposed, but an example thereof may include a yoke type inspection equipment as illustrated in FIG. 2. The inspection device is configured to include a yoke 30 which magnetizes the subject 60, a power supplier which applies a current to a coil wound around the yoke 30, a magnetic field sensing sensor 50, an analyzer which analyzes an output signal from the magnetic field sensing sensor, and the like.
The related art has used an ultrasonic inspection method to sort out inferior goods during a manufacturing process of a vessel. The ultrasonic inspection method is difficult to perform an accurate measurement when a inspection device does not firmly contact a surface of the vessel, such that it may not accurately inspect a curved portion of a dome having the highest possible occurrence of defect; however, the present invention use a non-contact method. The non-contact method may inspect the curved portion and accurately check whether a defect is present in the vessel, independent of the size of the vessel.